The present invention relates to a radio-frequency (RF) coil for exciting a plasma discharge, and, more specifically, to a novel RF coil for exciting a visible-light-producing plasma in a high-intensity discharge (HID) electrodeless lamp and having a shape with reduced blockage of the luminous flux from the discharge lamp.
It is now well known that visible light can be produced from a discharge plasma excited by RF current. The RF current is provided by a coil, generally exterior to the lamp in which the discharge is excited, which coil must not only have satisfactory coupling to the discharge plasma, but must also have low RF resistive loss and small physical size to allow the majority of the light, released from the discharge, to be utilized and not be blocked by the coil itself. The usual excitation coil is of a long solenoidal shape, being derived from the single solenoidal coils of copper tubing, regularly utilized with water cooling, for exciting plasma torches utilized in crystal growing, fiberoptics manufacture and the like.
Prior art, as exemplified by U.S. Pat. Nos. 3,860,854 (cup-shaped coil); 3,763,392 (short solenoid); 3,942,058 and 3,943,404 (small high-intensity discharge lamps at the end of coaxial cable), all have low optical efficacy and has coil losses which can be reduced.